The present invention relates to wire-type printing heads used in serial printers and operating on a principle of energy of deformation accumulated in a leaf spring under the effect of the magnetic energy of a permanent magnet with subsequent conversion of the above-mentioned energy of deformation into the energy of printing due to the electric current which is passed, in accordance with a data to be printed, through a coil to create an electromagnetic force cancelling the attractive force of the permanent magnet.
Many types of wire-type printing heads have been known in the past, one example of which is shown in Figs. 1 and 2 of the attached drawings.
FIG. 1 is a semi-sectional view of a known spring-loaded wire-type printing head, and FIG. 2 is a sectional view along line A--A of FIG. 1.
In the drawings, reference numeral 1 designates a disk-shaped rear yoke. Stacked on the peripheral surface of rear yoke 1 are a permanent magnet 2, an intermediate yoke 3, and an armature yoke 4. One end of a leaf spring 5 is rigidly clamped between armature yoke 4 and intermediate yoke 3. The leaf spring 5 extends radially inward, i.e., toward the center of the disk-shaped rear yoke 1.
Fixed to the free end of leaf spring 5 is an armature 6 which operates on its free end the base (rear end) of a printing wire 7 which is rigidly attached thereto. The tip (front end) of printing wire 7 is arranged so that it can project through a guide portion 8a of a wire guide 8.
Located in the central portion of rear yoke 1 is a core 9 which is surrounded by a coil 10.
Although there are a plurality of wires 7, armatures 6 respectively supporting the wires 7, leaf springs 5 respectively supporting the armatures 6, and cores 9 respectively associated with the armatures 6, only one of each is illustrated for simplicity of illustration.
Reference numeral 11 designates a center pole which forms a magnetic path for a magnetic flux generated by coil 10. Reference numeral 12 designates a magnetic path formed by permanent magnet 2.
When coil 10 in the above-described structure is not energized, the magnetic flux developed by permanent magnet 2 flows through magnetic path 12, i.e., passes through intermediate yoke 3, armature yoke 4, armature 6, core 9 and rear yoke 1 and then is closed back to permanent magnet 2. Because of the force of magnetic attraction between core 9 and armature 6, the above-mentioned armature 6 is attracted by core 9, so that leaf spring 5 is deformed into a loose S-shaped form, thereby accumulating the energy of deformation.
If under this condition, coil 10 is energized, the magnetic flux developed by coil 10 will overcome the magnetic force developed by permanent magnet 2. Therefore, armature 6 will be released from core 9. As a result, the energy of deformation accumulated in leaf spring 5 also will be released, spring 5 will return to its natural state, and armature 6 will turn around its fulcrum point formed by an outer edge (left edge in the cross section of FIG. 1) of core 9. As a result, the tip of printing wire 7, which is fixed to armature 6, will be ejected in the forward (upward as seen in the figure) direction through guide portion 8a and will print a dot forming part of a character or the like onto a printing medium through an ink ribbon (not shown) placed between the tip of the wire and the recording medium.
During the printing operation, the magnetic flux due to the coil 10 will tend to avoid the "difficult" or oppositely directed magnetic path 12, and will flow through "easy" magnetic path 13.
However, for reduction of an equivalent mass, the end of armature 6 fixed to the wire is so formed to have a minimum strength to withstand the impact force developed by printing. Thus, from the dynamic point of view, the mechanism should have as light a weight as possible. But then magnetic path 13 is insufficient.
Apart from the flow in the direction opposite to that in magnetic path 12, the demagnetization flux of coil 10 creates interferences by flowing through paths 14a, 14b formed by adjacent armatures 6II, 6III and cores 9II and 9III (FIG. 2).
These interferences can be eliminated only with installation of completely independent magnetic circuits for adjacent drive elements which, however, will make the construction extremely complicated.
Thus, the known wire-type printing heads have an inefficient path for the demagnetization flux developed by the coil, and until now the problem of magnetic interference in these devices has not yet been solved.